In vehicles (trucks or the like), when rear side axle, which is a driving axle, has two axles, a trunnion suspension is used as a suspension for the rear side axle (see, for example, U.S. Pat. No. 6,264,231). As shown in FIG. 4, there is a rubber mount type using a rubber mount in trunnion suspensions. FIG. 4 is a schematic side view showing a structure of a trunnion suspension 1. In the trunnion suspension 1, leaf springs 11 to 13 are laminated and used on axles 5 and 6 of tires 3 and 4 at a rear side.
As shown in FIG. 4, pads 14 are provided between both end portions of the leaf springs 11 and 12. A predetermined interval is provided between end portions of the leaf springs 12 and 13. When stroke length of the leaf spring 12 positioned under the leaf spring 13 is equal to or greater than a predetermined length, the end portion of the leaf spring 13 contacts that of the leaf spring 12, and starts stroking. Rubber mounts 21 and 22 having a cornice shape are provided between the leaf spring 11 and the axle 5 and between the leaf spring 11 and the axle 6.
U-shaped bolts 31 and 32 holding the leaf springs 11 to 13 are disposed at center portions of the leaf springs 11 to 13. Nuts 33 and 34 are fastened at leading end portions of the U-shaped bolts 31 and 32 on a pad 41 disposed on a surface of the leaf spring 13. Support portions of trunnion sheet 42 supporting the leaf springs 11 to 13 are disposed between bottoms of the U-shaped bolts 31 and 32 and a lower surface of the leaf spring 11. A trunnion shaft 43 is disposed at a lower side of the leaf spring 11 between the U-shaped bolts 31 and 32. The trunnion shaft 43 is fixed at a bracket 44 via a hole of a main body portion of the trunnion sheet 42. The bracket 44 is fixed on a vehicle body 2.
In this trunnion suspension 1, shocks are applied from a road surface to the tires 3 and 4, and the shocks are transmitted to the leaf springs 11 to 13 through the axles 5 and 6 and the rubber mounts 21 and 22. The leaf springs 11 to 13 rotate around the trunnion shaft 43, and are elastically supported by the rubber mounts 21 and 22 on the axles 5 and 6, so that shock transmission to the vehicle body 2 is relieved.
In the pads 14 between both end portions of the leaf springs 11 and 12, as shown in FIGS. 5A to 5C, recesses are formed at center portions of both surfaces, and surfaces of the recesses are flat surfaces 14A. The leaf springs 11 and 12 shown in FIG. 4 are disposed on the flat surfaces 14A of the recesses. Bolt holes 14B are formed at projections of both end portions of the pad 14, and bolts are penetrated into the bolt holes 14B and are fastened by nuts on plates (not shown in the Figure) of upper portions of the rubber mounts 21 and 22. In the action of the trunnion suspension 1, the leaf springs 11 and 12, which contact the flat surfaces 14A of the recesses of both surfaces of the pad 14, stroke in an upper direction shown in FIG. 4. In this case, upper surfaces of the leaf springs 11 to 13 are compression surfaces, and lower surfaces thereof are tension surfaces.
However, in stroking of the leaf spring 12, local wear may occur on the tension surface (the lower surface) of the leaf spring 12 due to a corner portion of the left end portion side (the center portion side of the leaf spring 12) of the flat surface 14A of the pad 14. Although the wear portion of the leaf spring 12 is not a weakest stress portion, as shown in FIG. 6, the wear portion may early initiate breakage of the leaf spring 12. FIG. 6 shows a condition of a breakage portion 12A at the right side end portion of the tension surface of the leaf spring 12, and reference numeral 12B denotes a starting point of breakage.